package chelper;

import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;
import java.util.Set;
import java.util.TreeSet;

import io.InputReader;
import io.OutputWriter;
import misc.SimpleSavingChelperSolution;


public class E extends SimpleSavingChelperSolution {

  public void solve(int testNumber, InputReader in, OutputWriter out) {
    wrapSolve(testNumber, in, out);
  }

  @Override
  public void solve(int testNumber) {
    int n = in.nextInt();
    String s = in.nextString();

    TreeSet<Integer> pacmans = new TreeSet<>();
    TreeSet<Integer> allStars = new TreeSet<>();

    for (int i = 0; i < n; i++) {
      char c = s.charAt(i);

      if (c == 'P') {
        pacmans.add(i);
      }

      if (c == '*') {
        allStars.add(i);
      }
    }

    int l = -1;
    int r = n * 10;

    while (r - l > 1) {
      int m = (l + r) / 2;

      TreeSet<Integer> stars = new TreeSet<>(allStars);

      for (Integer pacman : pacmans) {
        int L = pacman;
        int R = pacman;

        while (!stars.isEmpty()) {
          int k = stars.first();

          int L1 = Math.min(L, k);
          int R1 = Math.max(R, k);

          int dist = Math.min(pacman - L1, R1 - pacman) + R1 - L1;
          if (dist <= m) {
            stars.pollFirst();
            L = L1;
            R = R1;
          } else {
            break;
          }
        }


//        while (left > 0 && !stars.isEmpty()) {
//          int k = stars.first();
//
//          int dist = Math.abs(pos - k);
//
//          if (dist <= left) {
//            stars.removeAll(new TreeSet<>(stars.subSet(
//                Math.min(k, pos),
//                Math.max(k, pos) + 1)));
//
//            left -= dist;
//            pos = k;
//          } else {
//            break;
//          }
//        }
      }

      if (stars.isEmpty()) {
        r = m;
      } else {
        l = m;
      }
    }

    out.println(r);
  }
}